I wouldn't even remotely trust anyone at a gun show to give you an honest deal if you don't know exactly what you have. There is a ton of info on Lugers online and in print. If nothing else check out gunbroker just to see what is out there and general price ranges. I haven't checked recently on a Luger (I'm trying to build a WWII collection) but they seemed to start at around $900 and go on up to $20k depending on what it was.
As much as I would love to make an offer on your Luger I don't even remotely have the funds to even make an insulting offer. Good luck and keep us posted on what you find out about yours. I so need to win the lottery as I know where to get a pristine Johnson automatic as well. Friend of my parents who has had it forever and offered to sell it to me. MUST...WIN...LOTTERY!!!!
